[release-15.0] BugFix: Unsharded query using a derived table and a dual table (#12484) (#12492)

* BugFix: Unsharded query using a derived table and a dual table (#12484)

* test: add failing test for a query in an unsharded keyspace using a derived table and reference table

Signed-off-by: Manan Gupta <manan@planetscale.com>

* feat: fix logic to check if a single unsharded keyspace is used

Signed-off-by: Manan Gupta <manan@planetscale.com>

* feat: fix test output

Signed-off-by: Manan Gupta <manan@planetscale.com>

---------

Signed-off-by: Manan Gupta <manan@planetscale.com>

* feat: remove test files

Signed-off-by: Manan Gupta <manan@planetscale.com>

---------

Signed-off-by: Manan Gupta <manan@planetscale.com>

go/vt/vtgate/planbuilder/testdata/select_cases.json

@@ -7870,5 +7870,43 @@
         "main.dual"
       ]
     }
+  },
+  {
+    "comment": "query with a derived table and dual table in unsharded keyspace",
+    "query": "SELECT * FROM unsharded_a AS t1  JOIN (SELECT trim((SELECT MAX(name) FROM unsharded_a)) AS name) AS t2 WHERE t1.name >= t2.name ORDER BY t1.name ASC LIMIT 1;",
+    "v3-plan": {
+      "QueryType": "SELECT",
+      "Original": "SELECT * FROM unsharded_a AS t1  JOIN (SELECT trim((SELECT MAX(name) FROM unsharded_a)) AS name) AS t2 WHERE t1.name >= t2.name ORDER BY t1.name ASC LIMIT 1;",
+      "Instructions": {
+        "OperatorType": "Route",
+        "Variant": "Unsharded",
+        "Keyspace": {
+          "Name": "main",
+          "Sharded": false
+        },
+        "FieldQuery": "select * from unsharded_a as t1 join (select trim((select max(`name`) from unsharded_a where 1 != 1)) as `name` from dual where 1 != 1) as t2 where 1 != 1",
+        "Query": "select * from unsharded_a as t1 join (select trim((select max(`name`) from unsharded_a)) as `name` from dual) as t2 where t1.`name` >= t2.`name` order by t1.`name` asc limit 1",
+        "Table": "unsharded_a, dual"
+      }
+    },
+    "gen4-plan": {
+      "QueryType": "SELECT",
+      "Original": "SELECT * FROM unsharded_a AS t1  JOIN (SELECT trim((SELECT MAX(name) FROM unsharded_a)) AS name) AS t2 WHERE t1.name >= t2.name ORDER BY t1.name ASC LIMIT 1;",
+      "Instructions": {
+        "OperatorType": "Route",
+        "Variant": "Unsharded",
+        "Keyspace": {
+          "Name": "main",
+          "Sharded": false
+        },
+        "FieldQuery": "select * from unsharded_a as t1 join (select trim((select max(`name`) from unsharded_a where 1 != 1)) as `name` from dual where 1 != 1) as t2 where 1 != 1",
+        "Query": "select * from unsharded_a as t1 join (select trim((select max(`name`) from unsharded_a)) as `name` from dual) as t2 where t1.`name` >= t2.`name` order by t1.`name` asc limit 1",
+        "Table": "dual, unsharded_a"
+      },
+      "TablesUsed": [
+        "main.dual",
+        "main.unsharded_a"
+      ]
+    }
   }
 ]

go/vt/vtgate/semantics/semantic_state.go

@@ -372,7 +372,7 @@ func (st *SemTable) SingleUnshardedKeyspace() (*vindexes.Keyspace, []*vindexes.T
 	for _, table := range st.Tables {
 		vindexTable := table.GetVindexTable()
 
-		if vindexTable == nil || vindexTable.Type != "" {
+		if vindexTable == nil {
 			_, isDT := table.getExpr().Expr.(*sqlparser.DerivedTable)
 			if isDT {
 				// derived tables are ok, as long as all real tables are from the same unsharded keyspace
@@ -381,6 +381,13 @@ func (st *SemTable) SingleUnshardedKeyspace() (*vindexes.Keyspace, []*vindexes.T
 			}
 			return nil, nil
 		}
+		if vindexTable.Type != "" {
+			// A reference table is not an issue when seeing if a query is going to an unsharded keyspace
+			if vindexTable.Type == vindexes.TypeReference {
+				continue
+			}
+			return nil, nil
+		}
 		name, ok := table.getExpr().Expr.(sqlparser.TableName)
 		if !ok {
 			return nil, nil